滇东南三叠系沉积物源分析及对华南西南缘印支期碰撞造山作用的响应

The Indosinian tectonics and sedimentation in southwestern South China were thought to be related to the Paleo-Tethyan ocean closure and subsequent continental collision or result from the Pacific northwestward subduction. In the latter view, the Indosinian movement in the southwestern South China was considered tectonothermal events in the reactivated paleo-sutures, rather than a collisional orogeny between the South China and Indochina blocks. If there occurred an Indosinian collisional orogeny between the South China and the Indochina blocks, it should be confirmed that voluminous detritus were dispersed from this orogen into the Triassic Youjiang turbiditic basin,such as a variety of volcanic lithic fragments, late Paleozoic detrital zircons, detrital Cr-spinels derived from ophiolitic sources and detrital rutiles of eclogite- and/or granulite- facies metamorphic origin, reflecting the evolution from oceanic subduction to continental collision. This proposal will focus on the Triassic sedimentary sequences in the southeastern Yunnan Province, the western part of the Youjiang Basin, and plan to conduct a detailed sedimentary provenance analysis by imploying sandstone modal composition, detrital zircon geochronology and detrital single mineral (zircon, rutile and spinel) chemistry. Combined with the sedimentologcial study of the Triassic strata, these provenance data will be used to (1) constrain the major source areas, (2) distinguish the detrital record of the southwestern provenance and characterize its lithologic constitutes and tectonic nature, and then (3) reconstruct the Indosinian tectonic evolution of the southwestern South China.

对华南西南部印支期构造运动和盆地沉积主要存在两种不同的认识，或将其归为古特提斯洋关闭-碰撞造山体制，或认为其受太平洋俯冲影响。后者认为华南西南缘印支运动不是与印支地块间的碰撞造山作用，而是古缝合带的构造热活动。要确定华南西南缘是否存在印支期碰撞造山作用，关键在于能否在右江三叠系地层中找到来自西南物源区的、代表印支期碰撞造山作用的碎屑沉积记录，如火山弧来源的火山岩岩屑、晚古生代碎屑锆石，蛇绿岩剥蚀的碎屑Cr尖晶石和榴辉岩-麻粒岩相变质成因的碎屑金红石等，反映源区从大洋俯冲到陆陆碰撞的造山演化。本项目拟选择右江盆地西部滇东南三叠系进行砂岩碎屑组分和碎屑锆石、金红石及尖晶石单矿物物源分析，结合地层沉积学研究，（1）对该区三叠系沉积物源进行详尽解剖，（2）识别西南物源区的碎屑沉积记录，并据此判别该源区的岩石组成和构造性质，（3）进而揭示华南西南缘印支期构造演化及属性。

华南西南缘是否存在印支期俯冲-碰撞造山作用及其造山机制一直存在较大争议，由此产生滇东南八布基性/超基性岩是否为蛇绿岩、右江盆地晚二叠世-三叠纪盆地构造属性如何等诸多科学问题。为此在项目资助下，选择滇东南八布基性/超基性岩体周边广布的碎屑岩为研究对象，通过其物源分析揭示源区背景和区域构造演化。该碎屑岩系长期以来被认为是右江盆地三叠纪浊流沉积系统的一部分，可与桂北中三叠统沉积对比。为此，我们系统总结了桂北（-黔南）晚二叠世-三叠纪碎屑沉积物源数据，确定早三叠世盆地物源由晚二叠世的峨眉山火山岩转变为中三叠世已卷入古特提斯分支洋盆二叠纪火山弧的造山带源区，盆地由被动陆缘盆地转变为周缘前陆盆地，指示华南西南缘存在印支期俯冲-碰撞造山作用，并基于晚二叠世碎屑沉积深入剖析了峨眉山火山岩源区的风化剥蚀和岩浆过程，对沉积古地理和古气候环境进行约束。八布碎屑岩区域上夹有薄层硅质岩-硅质泥岩沉积，推测可与紧邻基性/超基性岩的含早二叠世放射虫化石的硅质岩层对比。基性/超基性岩西则的碎屑岩序列见有含火山岩砾石的砾岩和含火山岩岩屑的砂岩，在碎屑锆石年龄组明显与桂北晚二叠世-中三叠世砂泥岩不同，含有~285Ma的最年轻锆石年龄组，但与越北和哀牢山带早中二叠世碎屑岩相似。结合区域地层，认为所研究的八布碎屑岩沉积时代应为早二叠世而非前人认为的三叠纪。所含有的早二叠世锆石颗粒微量元素指示弧/造山型岩浆岩源区，应与华南西南缘古特提斯分支洋盆的俯冲岩浆活动有关。上述研究对重新认识华南西南缘印支期的构造演化、古特提斯洋的俯冲-闭合造山事件和东亚地区的拼贴历史具有重要意义。项目所取得如上成果也提出了新的科学问题，为进一步研究提供研究方向，例如八布碎屑岩是否可与哀牢山带的火山-沉积序列对比；为此我们已经在项目资助下在哀牢山带完成了初步的野外工作和样品数据处理，有望取得新的研究成果。